The present invention relates to a plant for feeding alternating voltage between places remotely located through a long first line, which comprises a feeding arrangement for feeding electric power to the first line.
The invention comprises all types of such plants, such as for example for transmitting alternating voltage through high voltage lines over long distances from places where it is generated to consumer networks. xe2x80x9cPlaces located remotelyxe2x80x9d and xe2x80x9clong first linexe2x80x9d mean that the distance is at least in the order of kilometres, but lines of this type extend in practice over hundreds of kilometres.
In order to illuminate the invention and the problem to be solved thereby the case of such a plant for a line for feeding alternating voltage to railway vehicles will hereinafter be described, although the invention, accordingly, is not at all restricted to this field of use, but the corresponding problem is also there in other possible fields of use of the invention.
In such plants in so-called railway supply a one-phase alternating voltage is fed to railway vehicles through said first line, and since the first line has a comparatively high impedance, both resistance and reactance, the power possible to transmit to the railway vehicles along the first line will be restricted as a consequence of the phase shift between the current and the voltage, voltage drops over the line and losses in the line.
Different solutions have been suggested for solving this problem with a limited transmittable power from the first line, which aim at reducing the impedance in the first line. Among these, lines with double supply and arrangement of different types of reinforcement lines running in parallel with the first line constituting the contact line may be mentioned. Another way to attack this problem is to arrange alternating voltage transmission systems in parallel with a higher voltage than the voltage of the first line. This means that the voltage is step-up transformed to a higher level and led in parallel with the first line, which means considerably lower losses, in which the voltage may for example be step-up transformed from about 16 kv in the first line to 130 kV in the parallel line. The voltage is then step-down transformed at different locations along the first line for feeding thereof. Completely separate high voltage transmission lines may so be used with a connection to the first line through transformers uniformly distributed or a use of autotransformer systems may take place. One of the disadvantages of this solution is the requirement of expensive transformers. All these solutions are comparatively costly.
The object of the present invention is to provide a plant of the type defined in the introduction, which enables an achievement of a good ability to transmit electric power from the first line to loads therealong in a simpler and by that less expensive way than through the plants already known discussed above.
This object is according to the invention obtained by providing said arrangement with a second line conducting direct voltage and adapted to extend along the first line and inverters arranged at locations along the extension of the lines and connected between the direct voltage line and the alternating voltage line for converting direct voltage into alternating voltage fed therethrough to the first line.
The feeding of electric power between different locations along the first line may by providing the feeding arrangement with a line conducting direct voltage take place through a direct current which will only feel the resistive part of the second line and the impedance and the voltage drop over the line will by that be lower. Furthermore, the current in the direct voltage line may without any problem be considerably lower than in direct transmission over the first line, since a considerably higher, namely 2{square root over (2)} times, direct voltage may be transmitted on the direct voltage line than the voltage level desired to be obtained on the first line without any need of any transformer for reducing the voltage to the level desired between the inverter and the first line. This will result in a substantially increased transmission capacity and reduced losses.
According to another preferred embodiment of the invention the feeding arrangement comprises a third line conducting alternating voltage and led to the first line for feeding electric power to the first line, and a rectifier is connected between the third and the second line. A supply line conducting alternating voltage and emanating from a source for generation of electric power may by this in the plant according to the invention in a conventional way be led to the first line for feeding power thereto, in which, however, a rectifying takes place through the rectifier for being able to provide an alternating voltage of a different type through an inverter, primarily with respect to the number of phases and the frequency, to the first line, in which the direct voltage produced by the rectifier of the second line conducting direct voltage here is utilised for a direct voltage transmission in the second line to locations where inverters are arranged and may transfer electric power to the first line.
According to another preferred embodiment of the invention the feeding arrangement comprises a converter with a rectifier connected to the third line, an inverter connected to the first line and an intermediate link arranged therebetween, and the second line is connected to the intermediate link for feeding direct voltage along the first line. By in principle prolonging the direct voltage line present between a rectifier and an inverter in a conventional converter in this way a simple way to transmit electric power along a line for feeding alternating voltage is obtained while maintaining a good transmitting capacity from the first line to possible loads and low losses along the very line.
According to another preferred embodiment of the invention the first line is a contact line for feeding one-phase alternating voltage to railway vehicles. The invention is particularly well suited for this application, in which it is important that sufficient power may be transferred to such a vehicle at different locations along the contact line.
According to another preferred embodiment of the invention the connection between said inverter and the first line has no transformer, which keeps the costs for the plant on a low level and still enables a comparatively weak current with small losses in the second direct voltage transmission line.
According to another preferred embodiment of the invention the inverter is a three-level inverter, i.e. an inverter having a neutral point, whereby a better curve shape is obtained for the alternating voltage provided to the first line by the inverter.
It is pointed out that the claim definitions xe2x80x9crectifierxe2x80x9d and xe2x80x9cinverterxe2x80x9d are to be given a broad sense and mean that in the direction intended, such as from the third line to the second line, a transition from alternating voltage to direct voltage and from direct voltage to alternating voltage, respectively, takes place, but this does not exclude the possibility to transmit power through these components in the opposite direction, so that these then function as inverter and rectifier, respectively, instead, which will be the case when a railway vehicle will brake.
Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims.